The device will not be as effective in 2 situations:
1- There is already a good amount of airflow in the duct before the installation.
If the room does not get to the desired temperature although you have a good flow already, the problem is somewhere else. for example bad windows or bad insulations.
2- There is almost no airflow coming from the vent before the installation.
This could be an indication of a disconnected duct. Please kindly note that the booster fan cannot generate heat and cool. It will boost whatever already coming from the vent. If the vent is somehow disconnected from the main system, adding a booster fan will not be very helpful.
In both cases, we recommend you consult with a contractor.